We propose to study the mechanisms of altered gas exachange in Acute Respiratory Failure (ARF) utilizing a newly developed multiple inert gas technique, and to compare the data with more conventional techniques which utilize oxygen and carbon dioxide transfer. The mechanisms identified will be correlated with the apparent etiology and/or clinical setting, e.g., shock, aspiration, etc. We will also assess the contribution of cardiac insufficiency to the marked hypoxemia of ARF. We will evaluate the effects of: 1. Stepwise increase in positive end expiratory pressure, and 2. Stepwise increases in tidal volume on the distribution of ventilation/perfusion (VA/Q) and cardiac output. The application of this new technique, while obtaining simultaneous data from standard techniques, to a large number of patients with ARF should provide valuable information concerning the mechanisms of altered gas exchange. A critical evaluation of currently used empiric forms of therapy; oxygen, PEEP, and increasing tidal volumes, each with inherent morbidity, should provide a more rational basis for therapy.